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Effects Of Restriction Of Wild Herbivore Movement On Woody And Herbaceous Vegetation In The Okavango Delta Botswana

Wildlife movements and migrations are increasingly being disrupted due to ongoing fragmentation of African savannahs (Fynn & Bonyongo, 2011). Free-ranging herbivores that are not subject to restrictions of movement by fences and other human land-use effects generally do not degrade their habitats because their highly mobile foraging strategies continually deflect their foraging to new areas and resources (McNaughton, 1985; Fynn, 2012). By contrast, overutilization of preferred forage by less mobile sedentary herbivores often results in habitat degradation and compositional shifts (Fynn, 2012). Continuous intense trampling and foraging by large herds of herbivores may have detrimental effects on biodiversity and ecosystem function, and consequently smaller herbivores (Laws, 1970; Fritz et al., 2002). At the same time, in migratory systems, large mobile herds of grazers provide nutrients (dung and urine additions) and improve light availability (trampling effects) in an intense, but transient manner, and can therefore stimulate grassland productivity and contribute to the maintenance of grassland dominated by short palatable species (Collins et al., 1998; Frank, Mcnaughton & Tracy, 1998; Fynn et al., 2005). This ecosystem engineering effect (Jones, Lawton & Shachak, 1994) is critical for maintaining essential habitat features, such as high forage quality and open, high visibility, lowpredation-risk short grassland (Smuts, 1978; Riginos & Grace, 2008; Hopcraft, Olff & Sinclair, 2010). It may also maintain processes that maximize nutrient and energy flow to grazers (Fynn, 2012). Similarly, it has been shown that intense browsing by large concentrations of browsers may modify the structure of the woody layer to create short, high-quality browsing lawns (Fornara & Du Toit, 2007) at various heights, thereby increasing nutrient and energy flow to smaller browsers (Skarpe et al., 2004; Makhabu, Skarpe & Hytteborn, 2006; Fornara & Du Toit, 2007; Valeix et al., 2011). Where heavy grazing or browsing is maintained all year as a result of habitat fragmentation and sedentization of large herbivores; however, destructive effects such as the replacement of productive palatable grasses and trees by unpalatable or less productive species resulting in a decline in ecosystem productivity may be expected (Whyte & Joubert, 1988; O’connor, Goodman & Clegg, 2007; Fynn, 2012). Consequently, understanding the effect of the restriction of large free-ranging herbivores’ movement may have important implications for effective environmental conservation.

In 1982/3, the Southern Okavango Buffalo Fence (Fig. 1) was erected towards the distal end of the Okavango Delta to separate Cape buffalo (Syncerus caffer), known carriers of foot-and-mouth disease, from the cattle that at the time formed an important part of Ngamiland District’s economy and cultural heritage (Campbell, 1976; Tlou, 1976; Ngamiland District Council, 2002; Government of Botswana, 2003). With the implementation of the 1991 Ngamiland Land Use Plan, this fence has also become a boundary separating different land use zones – wildlife management areas and tribal grazing areas. A time series of satellite imagery of the Okavango Delta reveals a noticeable reduction in the standing vegetative productivity of sandveld vegetation to the north of the Southern Buffalo Fence, relative to the south, where the fence crosses the sandveld tongue that lies between the seasonal Matshibe and Kweeni rivers (Fig. 2). Anecdotal evidence provided by wildlife veterinarians points to blocked migration
routes for zebras, but the heavy presence inside the fence of elephants (Loxodonta africana), along with visible tree damage, indicate that they might also be playing an important role (De Beer et al., 2006; Trollope et al., 2008). Certainly, field assessments show that both the herbaceous and woody vegetation layers at points inside the wildlife management area are more open in appearance than at points just across the fence on the tribal-grazing side (Gibbes et al., 2013). While the extensive wildlife system of northern Botswana still holds the potential for wildlife movements into neighbouring countries such as Zimbabwe,
Namibia and Angola, fences such as the Southern Okavango Buffalo Fence, which forms the southern boundary of the wildlife areas, form strong barriers for wildlife movement south of the Okavango Delta into the critical wetseason grazing of the northern Kgalagadi.

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